Arrangement for modulating grounded grid amplifiers



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ARRANGEMENT FOR MO'DULATINGl GROUNDED GRID AMPLIFIERS v Dennis Illingworth Lawson, Cambridge, England, assigner to Pye Limited, Cambridge, England,

a British company Application May 10, 1949, Serial No. 92,283 In Great Britain September 10, 1947 11 Claims.

In grounded grid triode amplifiers, the grid has to be maintained at earth potential so far as the radio frequencies are concerned and the present invention has for an object to provide a circuit whereby modulating potentials can be fed to the grid while still maintaining it at earth potential with regard to radio frequencies.

According to one aspect of the present invention, the grid of a grounded grid triode is coupled to earth through an open ended transmission line which is resonant at the radio frequency at which the valve is intended to operate, for example a quarter wave line. The quarter Wave line effectively maintains the grid at earth potential so far as the radio frequencies are concerned. Modulating potentials may be fed to the grid of the valve.

The invention may be applied to short wave transmitters in which the carrier frequency is amplified by a grounded grid triode. The modulation to be transmitted is then applied to the grid of the valve, the grid being coupled to earth through the quarter Wave transmission line. The quarter wave line has an impedance inverting property such that if one end is open circuited, the impedance across the other end will be zero. The modulation may comprise a television vwaveform or a part of a television waveform or an audio frequency Ior may consist of pulses or stepped potentials to control the output power from the valve.

The invention further consists in apparatus comprising a triode amplifying valve having a cathode, a grid and an anode, and at least one resonant circuit associated with the valve, Awherein the grid is coupled to earth through an open ended transmission line resonant at said frequency whereby the grid is maintained substantially at earth potential with regard to oscillations at that frequency.

The present invention also consists in radio transmitting apparatus comprising a radio frequency oscillator, a grounded grid triode amplifying Valve having a cathode, a grid and an anode, means for applying the output from said oscillator to said cathode, an open ended transmission line resonant at the frequency of said oscillator between said grid and earth to maintain said grid at earth potential with respect to the radio frequency oscillations applied to the' cathode, means for applying a modulating potential to said grid, and means for feeding themodulated radio frequency oscillations at said anode to a transmit-- ting antenna.

A feature of this invention consists in includ-A ing in the anode circuit of the valve a coaxial line, the inner conductor of which is connected to the anode and the outer member to earth.

According to a furtherl aspect the present invention consists in radio transmitting apparatus comprising a triode amplifying valve having a cathode, a` grid and an anode, means for applying a radio Yfrequency carrier to the cathode, an annular disc Yconnected to the grid and surrounding the valve, a second annular disc surrounding the valve insulated from the first annular disc and connected to earth, at least one co-aXial open ended transmission line resonant at the carrier frequency and comprising inner and outer members Aconnected to said annular discs, an inner tubular member connectedto the anode and an outer tubular member cro-axial with the inner tubular member and connected to the earthed annular disc, said inner and outer members forming a transmission lineresonant at the carrier frequency.

In order that the invention may be more fully understood reference will now be made to the accompanying drawings in Which- Figure 1 is a circuit diagram lolf a short wave radio transmitter incorporating a grounded grid amplifier according to this invention.

Figure 2 is a somewhat diagrammatic section of a constructional embodiment of the grounded grid amplifier.

Referring to Figure 1; a valve generally indicated by the reference l0, has a cathode Il, grid I2 and anode |37. Connected to the cathode I I is a resonant circuit I4 which is shown as cornprising inductance and capacitance. A radio frequencycarrier at the resonant frequency of the circuit |14 is fed to the cathode Il through a connection |15 from an oscillator or driver 22. In the anode circuit there is a further tuned resonant circuit I6 from which outputv may be taken from the connection II which leads to the transmitting antenna. operate under grounded grid conditions and this would normally involve the connection of the grid directly to earth but in accordance with the present invention the grid is coupled to earth by an open ended transmission line resonant at the operating frequency and comprising two conductors I8 and l9 each a quarter wavelength long at the loperating frequency, the conductor I8 being connected to the grid and the conductor I9 being connected to earth. B'y this means the grid I2 is maintained substantially at earth potential with regard to oscillations at the operating radio frequency but the grid I2 is not earthed by the transmission line in respect of other lower The valve is intended to 3 frequencies such for instance as direct current or audio frequency oscillations. In consequence, if a modulating potential is applied from a modulator 2l to the lead 20 connected to the grid l2 the output fromthe valve I0 is modulated.

As shown in Figure `2, the ltriode valve 10 is constructed withthe grid l2 extending in kthe form of a conducting annular disc 25 which surrounds the valve envelope. This disc 25 is spaced by insulating spacers 26 from the end of a metal cylinder 21 constituting the outerelement of a concentric line in the output circuit of the valve, the inner element 28 of the line being connected directly to the anode i3 ofthe valve l0. This inner element 28 may be tubular and may be provided with apertures 29 at its end adjacent the valve for enabling a stream of air to be blown therethrough'for cooling the anode i3. Disposed parallel to the output concentric line 21, 28 is a second concentric line, the outer element 30 vof which is in electrical connection ,with the outer cylindrical member 21 of the output concentric line 21, 28 and the inner element 3|- of which extends from the surrounding disk 25 connected with the grid l2. The length of this second concentric line 3G, 3|V is a quarte;` of the wavelength at which the valve is intended to operate and therefore beingopen ended, it effectively maintains the grid at earth potential for radio frequencies. 2 two such quarter wave.k lines are shown, but more than two may be used if desired. The cathode l'l is connected to a disc 34 which is connected to earth through a quarter wave line 32, 33, the ends of which are bridged. The radio frequency drive or input is applied to the cathode Il through the lead l5 from an oscillator or driver 22, and the modulating voltage to theY grid I2 through the lead 2li/from a modulator 2 I. The oscillator or driver 22 and modulator 2r may each include one or more valves.

It is to be understood that where reference is made to quarter wave resonant transmission In the construction shown in Fig.

tronic valve having a cathode, a grid and ari anode, means for applying a radio frequency carrier to the cathode, a co-aXial transmission line associated with the anode and resonant at the carrier frequency, at least one open ended 'quarter-wave transmission line resonant at the same frequency as the ap-plied carrier connected to the grid, and a modulator for applying modulating potentials to the grid.

5. A circuit according to claim 4, wherein each open ended transmission line connected to the grid is a co-axial transmission line.

6. Apparatus comprising a triode electronic Valve having a cathode, ra grid and an anode, means for applying a radio frequency carrier to the cathode, a resonant circuit associated with the anode resonant at the carrier frequency, an open ended transmission line resonant at the carrier frequency connected to the grid and coupling the grid to earth, and means for applying y a modulation potential to the grid.

lines these lines may, if desired', be made an odd multiple of a quarter wavelength long.

While particular embodiments have been described with reference' to the accompanying drawings, it is to be' understood that various modifications may be made withoutv departing from the scope of the invention.

I claim: I

l. An electric circuit comp-rising a triode electronic valve having a cathode," agrid and an anode, means for applying radio frequencyv oscillations to the cathode, means for'taking" output from the anode, and an open ended transmission line, resonant at the same frequency "as that of the applied oscillations, connected to the grid to maintain it substantially at earth potential with regard to oscillations at'that frequency.

2. An electric circuit comprising a triode electronic valve having a cathode, a grid and an anode, a vresonant circuit connected to the cathode, means for applying radio frequency oscillations to said Ycircuit at itsre'so'nantv frequency, a resonant circuitcoupled' to the anode and resonant at the same frequency as that of the cathode circuit, and an open Vended transmission line connected to the grid and resonant at the same frequency as the resonant circuits coupled' to the cathode and anode. 'n

3. A circuit accordingtovclaiml 2, comprising also a modulator connected'to the grid 'to supply modulating potentials thereto.

7. Apparatus comprising a triode electronic Valve having a cathode, a grid and an anode, an vannular member surrounding the valve connect- Y ed to the grid, a resonant circuit coupled to the cathode, means for applying a radio frequency carrier to the resonant circuit coupled to the cathode, an open ended transmission line connected to the grid and coupling the grid to earth to maintain it substantially at earth potential with regard to oscillations at the carrier frequency, and a resonant circuit associated with the anode and resonant at the carrier frequency.

8. Radio transmitting apparatus comprising a radio frequency oscillator a grounded grid triode amplifying valve having a cathode, a grid and an anode, means for applying the output from said oscillator to said cathode, an open ended transmission line resonant at the frequency of said oscillator coupled between said grid and earth to maintain said grid at earth potential with respect to the radio frequency oscillations applied tc the cathode, means for applying a modulating potential to said grid, and means for feeding the modulated radio frequency oscillations at said anode to a transmitting antenna. 9. Radio transmitting apparatus comp-rising a triode amplifying valve having a cathode, a grid and an anode, means for applying the radio frequency carrier to the cathode, an open ended transmission line connected to the grid and coupling it to earth to maintain the grid substantially at Aearth potential with regard to the carrier'frequency, means for applying a. modulating potential 'to' the grid', a co-'aX'ial resonant circuit including a tubular inner member connected to the anode andv a tubular outer member connected to earth, said inner member being apertured adjacent to the anode, and means for circulating cooling air through the tubular :inner and outer members of the .transmission lines associated with the anodel 'q 10. Radio transmitting apparatus comprising a triode amplifying valve having a cathode, a

grid and an anode, mean's'for applying a radio frequency carrier tothe cathode, an annular disc connected to the grid and surrounding the valve, a second annular disc surrounding the valve into earth, at least one co-aXial open ended transmission line resonant at the carrier. frequency and comprising inner and outer members connectedto said annular discs, an inner tubular y q r member yconnected 'toI the. anode andan outer 4. An electric circuit comp-rising a triode elec- `75 tubular member co-airial with the inner tubular member and connected to the earthed annular REFERENCES CITED disc said inner and outer tubular members form- The following references are of record m the ing a transmission line resonant at the carrier me of this patent: frequency.

11. Apparatus according to claim 10 compris- 5 UNITED STATES PATENTS ing a plurality of open ended transmission lines Number Name Date resonant'l at the carrier frequency around the 2,216,170 George Oct. 1, 1940 disc connected to the grid. 2,408,355 Turner Sept. 24, 1946 2,462,866 Hotine Mar. 1, 1949 DENNIS ILLINGWORTH LAWSON. 10 2,485,400 McArthur Oct. 18, 1949 

